Optical discs, which may be used in computing, sound reproduction, and video, store data as information-bearing features, commonly referred to as marks that are formed within a recording layer of the disc. This data is both read and written by illuminating the recording layer with a semiconductor laser or laser diode of an optical recordable disc drive. If the data is written properly, such that the information-bearing marks burned into the recording layer by the laser are the correct length, and distance apart, then the disc should be interchangeable on any CD or DVD multi-play drive.
In general, the relationship between the total output power of a semiconductor laser and the current flowing though it is very sensitive to temperature and manufacturing variations, and to parameter drift due to aging of the laser. For example, the characteristics of a laser are highly dependent on the temperature of the laser chip. One important characteristic is the effect of temperature on the relationship between the diode's optical output and the current. In this case, the optical output decreases as the operating temperature increases and, conversely the optical output increases as the operating temperature decreases.
At the same time, the quality of the information-bearing marks written on the disc is directly related to the light power incident on the recording layer, which is, in turn, strongly influenced by various effects, including imperfections of the disc itself. For example, the characteristics of the recording disc may change with variations in ambient temperature and humidity. In addition, the surface of the disc may be uneven, scratched, or contain dust or a fingerprint. Due to such effects, some of the light produced by the laser may be scattered, and thus becomes unavailable to heat the recording layer appropriately.
Due to such variables, optical drive recorders typically define laser power as a range of values, rather than a fixed number, and include a control algorithm for adjusting and regulating the laser power when writing to a recordable optical disc. Known methods for regulating the laser power to achieve a stable optical output from the laser while writing include automatic power control (APC), and running optimal power control (ROPC).
APC provides a laser drive current based on a photodiode feedback loop that monitors the optical output of the laser, i.e., the light power, and provides a control signal for the laser that maintains operation at a constant optical output power level. The laser power is regulated based on information provided during disc writing by a sensing element called a front monitor diode (FMD). The FMD signal (also called the forward sense, or FS signal) is proportional to the total light power produced by the laser, and it is typically influenced by changes in laser parameters attributable to temperature drift, aging, and other effects.
ROPC regulates laser power based on the light reflected from the disc, rather than the light leaving the laser. The light reflection from the disc is monitored, and based on the detected reflection, the laser power is adjusted accordingly. When imperfections on the surface of the disc, such as scratches and fingerprints, scatter the light produced by the laser, ROPC is a more favorable means of regulating the light power than APC, since the light reflected from the disc is more directly related to the mark formation process. However, due to various system-level reasons, ROPC is hard to implement in a manner that consistently achieves the desired result. One reason is that ROPC customarily uses a sensor comprising an analog circuit to perform an absorption measurement. Performing the absorption measurement, however, in real time while the laser is writing to the disc at high speed requires a very precise and expensive analog circuit.
Accordingly, a need exists for an improved system and method for controlling laser power while writing a recordable optical disc.